patch 9.1.0823: filetype: Zephyr overlay files not recognized

Problem:  filetype: Zephyr overlay files not recognized
Solution: detect '*.overlay' files as dts filetype,
          include syntax tests for DTS files
          (Xudong Zheng)

Reference:
https://docs.zephyrproject.org/latest/build/dts/howtos.html

closes: #15963

Signed-off-by: Xudong Zheng <7pkvm5aw@slicealias.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/input/dts_zmk.keymap b/runtime/syntax/testdir/input/dts_zmk.keymap
new file mode 100644
index 0000000..14fb210
--- /dev/null
+++ b/runtime/syntax/testdir/input/dts_zmk.keymap
@@ -0,0 +1,23 @@
+#include <behaviors.dtsi>
+#include <dt-bindings/zmk/keys.h>
+
+// Define macro for layers.
+#define LAYER_MAIN 0
+
+/ {
+	keymap {
+		compatible = "zmk,keymap";
+
+		layer_0 {
+			bindings = <
+				&kp Z
+				&kp M
+				&kp K
+				&zmk_string
+				&to LAYER_MAIN
+			>;
+		};
+	};
+
+	ZMK_MACRO(zmk_string, wait-ms = <10>; tap-ms = <10>; bindings = <&macro_tap &kp Z &kp M &kp K>;)
+};